In the manufacture of molded records, such as conventional audio records or the more newly developed video discs, a plastic material, such as a polyvinylchloride molding composition, is pressed between a pair of molding plates which are referred to in the art as stampers. The stampers have defined in their molding surfaces grooves which contain a surface relief pattern which is the negative of what is desired to be molded into the record.
The stampers are obtained by a process referred to as matrixing. In the matrixing process, the information desired to be molded in the record, such as audio programming or video programming, is initially recorded on magnetic tape. The recorded tape is then used to drive a cutter head which cuts a spiral groove into a recording substrate disc. The groove has defined along its length variations in its surface relief which correspond to the desired information. The recorded substrate is replicated in a series of electroplating steps. First, the recorded substrate is electroplated with a metal, such as nickel or silver, to provide what is referred to as a master. The master is then electroplated to provide what is referred to as a mold. The mold, thereafter, is electroplated to provide an electroformed negative of the recording substrate which is the stamper.
In the matrixing steps, extreme care is taken to prevent any deformation or damage of the grooved surface of the electroformed parts in order to maintain the fidelity of the replicated surfaces to the original recording made in the recording substrate.
In the matrixing process the face surface of the replicated part, that is the grooved surfaces, are formed with a bright, extremely smooth finish. The back surface, however, is considerably different in physical appearance. After electroforming, the back surfaces of the matrix parts generally exhibit a somewhat matte-like appearance. Furthermore, there are often surface defects such as somewhat rough areas. Still further common surface imperfections are referred to as nodules. The nodules are small protuberances on the back surface of the part, which typically are about 0.02 centimeter in diameter.
Stampers which have significant amounts of surface defects on their back surface are unsatisfactory for pressing records in general, and, particularly, for pressing of video discs. In the record molding process, the stampers are mounted on smooth, steel molding platens in a molding press. The stampers form the molding surface of the molding platens. In operation, the stampers are subjected to extremely high pressures and to cyclic heating and cooling. Under these molding conditions, the defects on the back surface of the stampers can cause deformation of the entire stamper with the surface imperfections on the back being effectively transferred through to the face of the stamper. This in turn causes a distortion of the information defined in the grooves of the stamper and in the resulting records molded on the stampers.
The problems caused by defects on the back surfaces of the stampers have been recognized in the record molding industry. In an attempt to prevent problems caused by the defects on the back surfaces of the stampers, it has heretofore been suggested to visibly examine the back surfaces and to either discard those with excessive defects or to hand sand the visible defects. It has been further suggested to mount the stampers on a turntable-like device and mechanically rotate the stamper while manually holding a pad of sandpaper, emery cloth, or the like against the stamper to remove the gross surface defects from the back surface of the stampers.
Hand sanding of the stamper backs has proven to be at best only marginally acceptable. The techniques heretofore employed are dependent to a large extent on the judgment of the operators as to what is required to correct a given stamper. A further problem that is encountered is that often the operator, by using excessive pressure or sanding for an excessive length of time, introduces additional defects such as circular grind marks or the like on the back, or produces ridges as a result of uneven sanding of the stamper backs.
The problems noted above have been recognized for many years in the audio record industry, but have remained generally unresolved in that the prior art processes provided marginally satisfactory results for most types of audio records. However, with the newer video discs, the problem of defects on the back surface of the stampers presents considerably greater problems and results in a substantial rejection rate of the stampers used for the manufacture of video discs. The signal elements on a video disc are generally orders of magnitude smaller than the equivalent signal elements on an audio record; and, furthermore, the grooves on the video record are likewise orders of magnitude narrower. Accordingly, the distortion caused by defects on the back of the stampers, which would not significantly affect the audio records stamper, have been found to cause major defects in the video discs. Problems such as lock groove, skip groove, signal dropout, and the like were found to be directly related to the distortions caused in the molded records by the surface imperfections on the back side of the molding stampers.
Hand sanding of the back surface of the stampers, as heretofore employed in the audio record industry, is not satisfactory for video discs. Defects are not corrected consistently and the effects of the operator-induced defects are emphasized to a much greater degree in the molding of the video discs.
Accordingly, it would be highly advantageous if an apparatus could be provided which could be accurately controlled so as to provide a uniform amount of grinding to the back surfaces of record stampers and thereby remove the surface defects without introducing additional defects.